1. Field of the Invention
This invention relates to an ignition timing control system for an internal combustion engine having means for prevention of knocking irrespective of the octane value of fuel used in the engine.
2. Prior Art
Generally, if a fuel having a low octane value is used in an internal combustion engine which is designed to operate on a fuel having a high octane value, knocking can occur with considerable frequency. To overcome this problem, an ignition timing control system for an internal combustion engine has been proposed by the assignee of the present application by Japanese Provisional Patent Publication (Kokai) No. 4- 101067, which comprises means for detecting occurrence of knocking, and means for retarding the ignition timing when occurrence of knocking is detected.
In the proposed control system, a plurality of control zones different in the ignition timing control characteristic according to the octane value of a fuel used are provided, by which the amount of advance of ignition timing is restricted under predetermined conditions, and the ignition timing is separately controlled in each of the control zones selected by an ignition timing retarding amount determined according to the degree of occurrence of knocking. When load on the engine exceeds a predetermined value while knocking does not occur, the use of the control zones is canceled by making a changeover from a control zone which is currently applied to a control zone suitable for a fuel having a higher octane value, i.e. to a control zone where the ignition timing can be set to a more advanced value, to thereby enable to control the ignition timing in a manner suitable to the octane value of the fuel used without causing hunting in the controlled ignition timing.
However, in the above proposed system, the predetermined value of load on the engine is set to such a high value that the changeover of the control zone is effected only when the engine is in a highly loaded condition where knocking can occur in the engine if a fuel having a low octane value is used. Therefore, when a vehicle on which the engine is installed is traveling at a high altitude where the load on the engine normally assumes values lower than the predetermined value and accordingly the load on the engine can scarcely exceed the predetermined value, no changeover of the control zone will be effected, once a control zone has been selected according to the ignition timing retarding amount, which results in degraded driveability. More specifically, while the ignition timing retarding amount is calculated so as to secure a required level of driveability, and ignition timing control is effected based on the calculated retarding amount in control zones suitable for respective octane values of the fuel used selected by the retarding amount, it is impossible to change a control zone once the control zone has been selected. This brings about a problem of degraded driveability when a wrong or inappropriate control zone is erroneously selected.
To solve this problem, one would be propose to set the predetermined value of the load on the engine to a lower value. However, if the predetermined value is simply set to a lower value, this results in another inconvenience that the frequency of changeover of the control zone increases, causing hunting in the controlled ignition timing when a fuel having a low octane value is used.
Further, in the above proposed system, control zones are selected irrespective of load on the engine. Therefore, if a control zone is erroneously selected when the engine is operating in a low load condition in which knocking can scarcely occur, the ignition timing may be controlled in a control zone corresponding to a low octane value even if a fuel having a high octane value is used, so that desired engine output cannot be obtained. Moreover, once the control zone has been selected, it is not canceled unless the load on the engine exceeds the predetermined value, which prevents ignition timing control from being carried out in a desired manner.